The cytoskeleton of skeletal and cardiac muscle cells is unique compared to that of all other cells. It consists of a nearly crystalline array of closely packed cytoskeletal proteins called the sarcomere. The sarcomere is elegantly organized as an interdigitating array of thin and thick filaments. The thick filaments are composed of myosin, the motor protein responsible for transducing the chemical energy of ATP hydrolysis into force and directed movement. The thin filaments are composed of actin monomers arranged in a helical array. There are four regulatory proteins bound to the actin filaments, which allows the contraction to be modulated by calcium ions. An influx of intracellular calcium initiates muscle contraction; thick and thin filaments slide past each other driven by repetitive interactions of the myosin motor domains with the thin actin filaments.
Of the thirteen distinct classes of myosin in human cells, the myosin-II class is responsible for contraction of skeletal, cardiac, and smooth muscle. This class of myosin is significantly different in amino acid composition and in overall structure from myosin in the other twelve distinct classes. Myosin-II forms homo-dimers resulting in two globular head domains linked together by a long alpha-helical coiled-coiled tail to form the core of the sarcomere's thick filament. The globular heads have a catalytic domain where the actin binding and ATPase functions of myosin take place. Once bound to an actin filament, the release of phosphate (cf. ADP-Pi to ADP) signals a change in structural conformation of the catalytic domain that in turn alters the orientation of the light-chain binding lever arm domain that extends from the globular head; this movement is termed the powerstroke. This change in orientation of the myosin head in relationship to actin causes the thick filament of which it is a part to move with respect to the thin actin filament to which it is bound. Un-binding of the globular head from the actin filament (Ca2+ regulated) coupled with return of the catalytic domain and light chain to their starting conformation/orientation completes the catalytic cycle, responsible for intracellular movement and muscle contraction.
Tropomyosin and troponin mediate the calcium effect on the interaction on actin and myosin. The troponin complex is comprised of three polypeptide chains: troponin C, which binds calcium ions; troponin I, which binds to actin; and troponin T, which binds to tropomyosin. The skeletal troponin-tropomyosin complex regulates the myosin binding sites extending over several actin units at once.
Troponin, a complex of the three polypeptides described above, is an accessory protein that is closely associated with actin filaments in vertebrate muscle. The troponin complex acts in conjunction with the muscle form of tropomyosin to mediate the Ca2+ dependency of myosin ATPase activity and thereby regulate muscle contraction. The troponin polypeptides T, I, and C, are named for their tropomyosin binding, inhibitory, and calcium binding activities, respectively. Troponin T binds to tropomyosin and is believed to be responsible for positioning the troponin complex on the muscle thin filament. Troponin I binds to actin, and the complex formed by troponins I and T, and tropomyosin inhibits the interaction of actin and myosin. Skeletal troponin C is capable of binding up to four calcium molecules. Studies suggest that when the level of calcium in the muscle is raised, troponin C exposes a binding site for troponin I, recruiting it away from actin. This causes the tropomyosin molecule to shift its position as well, thereby exposing the myosin binding sites on actin and stimulating myosin ATPase activity.
Human skeletal muscle is composed of different types of contractile fibers, classified by their myosin type and termed either slow or fast fibers. Table 1 summarizes the different proteins that make up these types of muscle.
TABLE 1Muscle Fiber TypeFast skeletalSlow SkeletalMyosin Heavy ChainIIa, (IIb*), IIx/dCardiac βTroponin I (TnI)TnI fast SKTnI slow SKTroponin T (TnT)TnT fast SKTnT slow SKTroponin C (TnC)TnC fast SKTnC slow/cardiacTropomyosinTM-β/TM-α/TPM 3TM-β/TM-αs*MHC IIb is not expressed in human muscle but is present in rodents and other mammals.
In healthy humans most skeletal muscles are composed of both fast and slow fibers, although the proportions of each vary with muscle type. Slow skeletal fibers, often called type I fibers, have more structural similarity with cardiac muscle and tend to be used more for fine and postural control. They usually have a greater oxidative capacity and are more resistant to fatigue with continued use. Fast skeletal muscle fibers, often called type II fibers, are classified into fast oxidative (IIa) and fast glycolytic (type IIx/d) fibers. While these muscle fibers have different myosin types, they share many components including the troponin and tropomyosin regulatory proteins. Fast skeletal muscle fibers tend to exert greater force but fatigue faster than slow skeletal muscle fibers and are functionally useful for acute, large scale movements such as rising from a chair or correcting falls.
Muscle contraction and force generation is controlled through nervous stimulation by innervating motor neurons. Each motor neuron may innervate many (approximately 100-380) muscle fibers as a contractile whole, termed a motor unit. When a muscle is required to contract, motor neurons send stimuli as nerve impulses (action potentials) from the brain stem or spinal cord to each fiber within the motor unit. The contact region between nerve and muscle fibers is a specialized synapse called the neuromuscular junction (NMJ). Here, membrane depolarizing action potentials in the nerve are translated into an impulse in the muscle fiber through release of the neurotransmitter acetylcholine (ACh). ACh triggers a second action potential in the muscle that spreads rapidly along the fiber and into invaginations in the membrane, termed t-tubules. T-tubules are physically connected to Ca2+ stores within the sarcoplasmic reticulum (SR) of muscle via the dihydropyridine receptor (DHPR). Stimulation of the DHPR activates a second Ca2+ channel in the SR, the ryanodine receptor, to trigger the release of Ca2+ from stores in the SR to the muscle cytoplasm where it can interact with the troponin complex to initiate muscle contraction. If muscle stimulation stops, calcium is rapidly taken back up into the SR through the ATP dependent Ca2+ pump, SERCA.
Muscle function can become compromised in disease by many mechanisms. Examples include the frailty associated with old age (termed sarcopenia) and cachexia syndromes associated with diseases such as cancer, heart failure, chronic obstructive pulmonary disease (COPD), and chronic kidney disease/dialysis. Severe muscular dysfunction can arise from neuromuscular diseases (such as Amyotrophic Lateral Sclerosis (ALS), spinal muscular atrophy (SMA) and myasthenia gravis) or muscular myopathies (such as muscular dystrophies). Additionally, muscle function may become compromised due to rehabilitation-related deficits, such as those associated with recovery from surgery (e.g. post-surgical muscle weakness), prolonged bed rest, or stroke rehabilitation. Additional examples of diseases or conditions where muscle function becomes compromised include peripheral vascular disease (e.g., claudication), chronic fatigue syndrome, metabolic syndrome, and obesity.
Accordingly, there is a need for the development of new compounds that modulate skeletal muscle contractility. There remains a need for agents that exploit new mechanisms of action and which may have better outcomes in terms of relief of symptoms, safety, and patient mortality, both short-term and long-term and an improved therapeutic index.